As the electric power generation utilizing natural energy, water power generation and wind power generation have been conventionally carried out. The water power generation typically utilizes the energy of water falling from a dam. In this case, a water-level difference needs to be maintained, and an installation condition is limited. In contrast, each of a tidal current, an ocean current, a comparatively large river, and the like have a large flow of water (hereinafter simply referred to as “water flow”), and there is the idea that electric power can be generated by utilizing the natural energy of the water flow. For example, in the case of the tidal current, there are places where large flows occur by the ebb and flow of the tide, and there is the idea that the electric power generation utilizing the water flow can be carried out at such places.
One example of a technology of generating the electric power by utilizing the tidal current as above is that a hollow supporting rod is provided at a fixed position under the sea, a screw blade configured to rotate by receiving the tidal current of an upper portion of the sea and a screw blade configured to rotate by receiving the tidal current of a lower portion of the sea are provided on the supporting rod, and an electric power generator in a station provided above the sea level is driven by rotational forces of the screw blades rotated by both tidal currents of the upper and lower portions to generate electric power (see Document 1, for example).
Moreover, one example of a technology of generating the electric power by utilizing the water flow is that a stake is planted on a bottom ground of a river, and a floating body including an electric power generating device having a rotating body (screw blade) configured to rotate by receiving naturally falling water flow is provided at the stake so as to be able to move up and down (see Document 2, for example).
Document 1: Japanese Laid-Open Patent Application Publication No. 2002-257023
Document 2: Japanese Laid-Open Patent Application Publication No. 2004-169564